Variable inductive device



p 1 3, 1963 s. w. BABCOCK ETAL 3,087,131

VARIABLE INDUCTIVE DEVICE Filed D80. 22, 1961 6m L WEE-(mu 12 wk 0 O O m 4 w INDUCTANCE- L (Mlcrohcnriu) STEPHEN W. BABCOCK DENIS W. G2\SEL.

ATTORNEYS.

iifii ihldl. Patented Apr. 23, 1963 3,687,131 VARIABLE INDUCTIVE DEV ICE Stephen W. llabcoclr, Poway, and Denis W. Grisel, La

.l'olla, Calif, assignors to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware Filed Dec. 22, 195i, Ser. No. 161,539 Claims. (Cl. 336-133) This invention relates in general to inductive devices and more particularly, to inductive devices wherein there are provided mechanical means for varying the inductance.

Conventional variable inductive devices commonly employ straight cylindrical coil assemblies with a considerable air gap. Variation of inductance is accomplished mechanically by means of a slug or core, usually of powdered iron, moved axially of the coil assembly. This construction necessarily limits the amount of inductance which may the achieved as well as the obtainable range of inductance variation. Further, in order to achieve such variation as is obtainable, a considerable axial travel of the core or slug is required necessitating extra installation space. Where space and weight are limited, this is an important consideration. In such conventional devices it is also important that tolerances be carefully maintained between the core and the coil assembly in order that uniformity of inductance variation responsive to the me chanical movement of the "core may be assured. Where vibration, shock, and extreme temperature and pressure variations are apt to be encountered, there is substantial difficulty in maintaining suitable tolerances.

The inductive devices of this invention largely eliminate or reduce the above objections to conventional devices by employing simple, effective and reliable mechanical means for varying in easily controlled fashion the inductance of the device, whether it be in the form of an inductor, a transformer, or other.

Accordingly, the invention has as an object the provision of an inductive device having wide application to installations "wherein there is desired a means for mechanically varying the inductance.

Another object is the provision of an inductive device which affords a maximum of available inductance and provides for a wide range of variation therein by mechanical means and which, through simple, rugged construction, assures reliability and uniformity of inductance variation over said range.

A further object is the provision of a device of the character described, which otters a maximum of inductance variation with a minimum of space and weight and which is accordingly well adapted to miniaturization techniques.

Still further objects and advantages of this invention will become apparent from the description which follows, when taken in conjunction with the claims and the drawing in which:

FIGURE 1 is an exploded view of the various components of the device as made up ready for final assembly;

FIGURE 2 is a view in longitudinal cross section of the assembled device;

FIGURE 3 is a view in plan, partially broken away to disclose interior details; and

FIGURE 4 is a graph of inductance values obtained with one particular embodiment of the device.

Turning now to FIGURES 1 and 2 of the drawing, the illustrated invention embodiment is generally designated by the numeral 6, being shown as comprised of a cylindrical housing 8 having an aperture or bore 10 extending centrally therethrough with a threaded upper end portion 12. A pair of diametrically opposed notches 14 are provided in the lower portion of the housing sidewall. Brass has been found a satisfactory material for the housing, although other materials of a non-magnetic character, as for example, aluminum, may be utilized. It should be noted that where conductive materials are used they offer the advantage of electrostatic shielding.

A permanent magnet 16 is disposed in rather close fitting relation with housing 8 in the aperture 10. Magnet 16 is of cylindrical cup shape with a centrally disposed cylindrical cavity 18 and upwardly opening diametrically opposed notches 2 -1} in its upper sidewall portions. There are thus defined sidewall portions of segmental cylindrical shape which extend upwardly from a base portion 22. Permanent magnet 16 is so magnetized as to provide a north magnetic pole in one such portion and a south magnetic pole in the other. The path of the flux lines of magnetic force emanating from these poles extends through base portion 22 and across the gap between the poles. As shown in FIGURES 2 and 3, the magnet 16 is retained in housing aperture 10 with the pairs of notches M and 2t) aligned to define conductor passageways and with the lower surface of base portion 22 substantially flush with the lower extremities of housing 8. An epoxy resin adhesive 24 has been found satisfactory for fixing the permanent magnet in position within the housing and serves at the same time to effect a moisture barrier between the housing and the magnet. Other adhesives may of course be used, as well as other means, as for example, soldering or brazing, for effecting fixing of the permanent magnet 16 within the housing. A wide variety of hard magnetic materials, i.e., materials capable of retaining their magnetization over long periods of time, are available for use in fabricating the permanent magnet 16. However, a material which has been found particularly satisfactory in this regard is a barium-iron ferrite ceramic sold under the trade name Arnox by Arnold Engineering Company, a subsidiary of Allegheny Ludlum Steel Corporation.

An inductive unit, generally designated 26, is centrally positioned within the permanent magnet cavity 18 in spaced relation with the cavity-defining magnet walls. The unit 26 is comprised of a continuous core 2% of annular ring shape and of rectangular cross section. Uniformly wound about the core 28 is an insulated currentcarrying conductor 38 the ends of which are passed for external connection through the passageways defined by the aligned notches 14 and 20. A silicone rubber is utilized to provide seals 32 and 34 in notches 14 and 20 to encircle conductor 3%. This sealant material is particularly satisfactory in that it accommodates the thermal contraction and expansion of the permanent magnet and the housing walls. Optimum results are obtained if the core is uniformly wound and accurately spaced centrally of the permanent magnet in order that a maximum of the permanent magnet lines of flux will normally pass through the core. The inductive unit 26 is arranged such that distribution of the permanent magnet flux with respect to the coil is balanced or symmetrical, eliminating harmonic distortion of an A.-C. signal applied to the de vice. Although not shown, a potting material may be utilized for fixing the inductive unit core in such position in cavity 18. For this purpose polyurethane resins are particularly satisfactory. Notwithstanding that the core is indicated as of annular shape, other continuous core configurations may be utilized, as may cross sections other than the rectangular one shown. For convenience of fabrication and optimum results, however, the illus trated core shape is to be preferred.

With respect to materials for the inductive unit, the core should be of a soft magnetic material, as for example, ferrite. A particularly satisfactory material has been found to be that sold by the General Ceramics Division c3 of Indiana General Corporation under the trade names Fer-ramic Q1, *Ferramic Q2, or Ferramic Q3. Althrough a variety of wires may be used for the core winding, Teflon insulated copper magnet wire has proven satisfactory.

A pole piece or shunt 36 of disc shape is threaded about its periphery for threaded receipt and retention within and movement through the upper portion 12 of the aperture 10. Provided in the upper face of the disc is a transverse slot 38 for reception of a tool (not shown) to effect rotation of the pole piece and pole piece movement toward and away from the permanent magnet 16 positioned in the lower portion of the aperture. Pole piece 36 serves as a shunt means for bypassing a portion of the permanent magnet flux from a normal path traversing cavity 18 and cutting across core 28. Accordingly, the material of the pole piece should be of low magnetic reluctance. Soft iron is particularly satisfactory for such application.

Describing now the operation of the above-described invention embodiment, lines of magnetic flux emanate from the permanent magnet 16 for which a flux path is provided through magnet base portion 22. The complete circuit negotiated by the lines of flux of course normally includes a path between the north and south poles directly across the cavity 18 and the core 28. Assuming for the moment that pole piece 36 is withdrawn from aperture 10, the permeability characteristics of the core will reflect the superimposition thereon of the flux from the permanent magnet, with the path of flux lines directed as above indicated. For purposes of this discussion it is assumed that the soft magnetic core 28 will be operating substantially below saturation, a situation which may be assured by proper selection of the core material. Under such conditions, a minimum of incremental inductance is offered to an A.-C. or a varying D.-C. current carried in the conductor 30 as a result of the incremental permeability of core 28. With pole piece 36 removed, the incremental permeability of the core is maintained at a minimum due to passage of the greater portion of the lines of magnetic force through the core. By then threading pole piece 36 into aperture 10, a portion of these lines may be shunted around the core and the incremental permeability increased accordingly. The extent of such shunting depends upon the proximity of the shunt or pole piece 36 to permanent magnet 16 which may be adjusted by simply mechanically rotating the threaded pole piece, as by inserting an appropriate tool (not shown) in slot 38 to achieve the desired position and proper flux shunt. Once suitably adjusted, the pole piece 35 may be easily secured in position, as by a small amount of paint at a perimeter location, which serves also as an indication of the maintenance of a fixed position.

The use of a pole piece coacting with the permanent magnet 16 in the manner indicated, provides a simple rugged construction capable of sustaining considerable vibration and shock without adverse effect. A large variation in the inductance of the device may be effected within the range of movement of the pole piece without a problem of alignment of the moving parts as would otherwise be the case were a movable core utilized. Because of its simple, compact structure, it is readily susceptible to miniaturization, particularly in view of the relatively small amount of linear travel necessary to effect substantial variation in the incremental inductance and the small envelope which, as a result, may be used to embrace the overall device.

The effectiveness of this invention is well illustrated in the graph of FIGURE 4. The device therein reflected is not to be construed, however, as a limitation of the invention. This particular device was constructed, as above described, with the housing 3 of brass, with a .300 in. outside diameter and a height of .200 in. The smooth lower portion of aperture was of .250 in. diameter. Threads provided in the threaded upper portion of the aperture were %"48.

Permanent magnet 16 was constructed of the material, Arnox, heretofore described, with an outer diameter of .230 in. and an overall height of .125 in. Cylindrical cavity 13 was of .150 in. diameter and .075 in. depth.

The inductive unit was comprised of a Ferramic Q2 core which was stabilized through application of a small amount of permanent D.-C. magnetization, thus reducing variations in performance due to temperature changes. The core was of rectangular cross section, having an outer diameter of .100 in., and inner diameter .070 in., and a depth of .030 in. One hundred and six turns of 38 QT copper wire, i.e., wire quadruple coated with a Teflon insulation, were uniformly wound about the core.

The pole piece 36 was comprised of soft iron, with an outer diameter of .230 in., a thickness of .050 in., and it was provided with a top-traversing slot 33 of .050 in. width and .025 in. depth.

In the FIGURE 4 graph the various curves have been plotted with inductance as an ordinate and temperature as an abscissa. The curves are numbered successively from top to bottom (1) through (9) and respectively illustrate the following A.-C. flux values and gap settings, the latter being measured from the lower extremity of the pole piece to the adjacent upper extremity of the permanent magnet. The frequency of the A.-C. current carried by the inductively wound coil Was one megacycle and the total of the A.-C. and permanent magnet flux was in all cases below that of maximum core permeability and core saturation.

A.-C. flux density gap (in.)

(gauss) The FIGURE 4 curves indicate the substantial range of inductance variation available from relatively limited travel of the pole piece relative to the permanent magnet. The high value of available inductance results from the use of the coil wound on a closed path magnetic core instead of on the customary straight cylindrical assembly, previously described, with its attendant considerable air gap. By judicious choice of materials for the core, permanent magnet, and the remaining components, good inductance stability as a function of ambient temperature and signal level were achieved. Linearity and resolution were also satisfactory with the materials chosen. In this connection it should be noted that the electrical characteristics of the device are largely dependent upon the hard and soft magnetic materials used for the permanent magnet and the core, respectively, and that by suitable selec tion of these, a wide variety of electrical characteristics may be achieved.

Although the invention has been described with respect to a preferred embodiment, it is to be understood that it may be altered, changed, or modified within the scope and spirit of the appended claims. Thus, although the invention has been described with respect to an inductor, it is likewise applicable to other inductive devices as, for example, transformers.

We particularly point out and distinctly claim as our invention:

1. A mechanically variable inductive device comprising,

in combination:

a housing having a bore portion of which is threaded; a permanent magnet having a north magnetized portion of segmental cylindrical shape and a south magnetized portion of segmental cylindrical shape interconnected at one end and spaced apart at the other an inductive unit comprising an annular ring core of magnetic material wound with an electrical currentthreaded portion adjacent said permanent magnet furnishing a shunt path of low reluctance to passage of the flux between said poles and upon rotation, threadedly movable toward and away from the percarrying conductor positioned in said permanent 5 manent magnet to determine the flux shunted and magnet gap for traversal by said flux and the superthereby the density of the permanent magnet flux imposition thereof upon the core to vary the insuperimposed upon the core, the core incremental cremental permeability of the core responsive to permeability, and the incremental inductance of the variations in the density of said flux; and vic shunt pole piece of disc shape threadedly retained 4. A mechanically variable inductive device compriswithin said bore threaded portion adjacent the gap formed in said permanent magnet furnishing a path of low magnetic reluctance for shunting the flux from said core and upon rotation, threadedly movable toa shunt pole piece threadedly retained in said bore threaded portion adjacent said permanent magnet furnishing a path of low magnetic reluctance for ing, in combination:

a housing having a bore with a threaded portion; a permanent magnet having a north magnetic pole and a south magnetic pole disposed in said housing in d d away f h permanent magnet t d ter. spaced apart relation with the path of flux between mine the flux shunted and thereby the density of the Said poles traversing said bore; permanent magnet flux u i d n th core, an inductive unit comprising a continuous core of magthe core incremental permeability, and the incremenn ic m rial w n with an elec rical rr n rytal indu tan e of the de i ing conductor positioned intermediate said north and 2. A mechanically variable inductive device compris- South magnetic poles in the P of flux travel thereing in combination; between for superimposition of said flux upon the a housing of non-magnetic but electrically conductive Core to ry the r m a p rm ability of t e Core material having a here ith a th d d i responsive to variations in the density of said flux; a permanent magnet having a north magnetic pole and and a outh magnetic l iti d at i h id f a a shunt pole piece threadedly retained within said bore cavity formed therebetween, said magnet being disthreaded Portion adjacent Said Permanent magnet posed in said housing in electrically conductive relafurnishing a P of 10W magnetic rehletahee for tion therewith and such that the paths of flux between shunting Said him from said core and p rotation, said poles traverse said bore and said cavity; threadedty movable toward and y if 0111 the P an inductive unit comprising an annular ring core of maheht magnet to determine the thlX shunted and magnetic material wound with an electrical currentthereby the density of the Permanent magnet flux carrying conductor positioned in said cavity for Superimposed P the the COT e incremental traversal by said flux and the superimposition therepermeability, and the incremental inductance of the of upon the core to vary the incremental permeability devlce. of th core responsive to variations in the density f 5. A mechanically variable inductive device comprising, in combination:

a housing; a. permanent magnet having a north magnetic pole and a south magnetic pole with a gap therebetween trashunting said flux fromsaid core and upon rotation, 40 versed by the path of travel of the flux between said threadedly movable toward and away from the per- P manent magnet to d t i th fl shunted and an inductive unit comprising a continuous core of maghereby the density of the permanent magnet fl netic material wound with a current-carrying conducsuperimposed upon the core, the core incremental tor Positiohed intermediate Said P0168 in p permeability and the incremental inductance of the said P of flux travel for superimposttloh of Sald device.

flux upon the core to vary the incremental permeability of the core responsive to variations in the den- 3. A mechanically variable inductive device comprising, in combination:

a housing having a bore therein, at least a portion of a sity of said flux; and shunt pole piece positioned adjacent said permanent which is threaded; magnet furnishing a path of low magnetic reluctance a permanent magnet h i a north magnetic 1 and for shunting said flux from said core and movable a south magnetic pole, said magnet disposed in said toward and y from the Permanent magnet t housing with the flux path between said poles traverstefmihe the flux shunted and thereby the t' y of ing said bore; the flux superimposed upon the core, the core morean inductive unit com i in an annular i core f mental permeability, and the incremental inductance magnetic material wound with an electrical currentof the device- CaIKYiHgtE HdHCtOE posiiioned igtermgldiateaidtnortli References Cited in the file of this patent an sou magne 10 p0 es in e pa 0 uX rave between said poles for superimposition of said dlux UNITED STATES PATENTS upon the core to vary the incremental permeability 2,180,850 Reiser Nov. 21, 1939 of the core responsive to variations in the density of 2,762,020 Gordon Sept. 4, 1956 said flux; and 2,899,654 Geiser Aug. 11, 1959 a shunt pole piece threadly retained within said bore 2,975,384 Geiser Mar. 14, 1961 

5. A MECHANICALLY VARIABLE INDUCTIVE DEVICE COMPRISING, IN COMBINATION: A HOUSING; A PERMANENT MAGNET HAVING A NORTH MAGNETIC POLE AND A SOUTH MAGNETIC POLE WITH A GAP THEREBETWEEN TRAVERSED BY THE PATH OF TRAVEL OF THE FLUX BETWEEN SAID POLES; AN INDUCTIVE UNIT COMPRISING A CONTINUOUS CORE OF MAGNETIC MATERIAL WOUND WITH A CURRENT-CARRYING CONDUCTOR POSITIONED INTERMEDIATE SAID POLES IN SAID GAP IN SAID PATH OF FLUX TRAVEL FOR SUPERIMPOSITION OF SAID FLUX UPON THE CORE TO VARY THE INCREMENTAL PERMEABILITY OF THE CORE RESPONSIVE TO VARIATIONS IN THE DENSITY OF SAID FLUX; AND A SHUNT POLE PIECE POSITIONED ADJACENT SAID PERMANENT MAGNET FURNISHING A PATH OF LOW MAGNETIC RELUCTANCE FOR SHUNTING SAID FLUX FROM SAID CORE AND MOVABLE TOWARD AND AWAY FROM THE PERMANENT MAGNET TO DETERMINE THE FLUX SHUNTED AND THEREBY THE DENSITY OF THE FLUX SUPERIMPOSED UPON THE CORE, THE CORE INCREMENTAL PERMEABILITY, AND THE INCREMENTAL INDUCTANCE OF THE DEVICE. 